The method relates to a method of drying a fiber web by an apparatus comprising two endless bands that are impermeable to air, first turning rolls around which the first band is arranged to turn, and second turning rolls around which the second band is arranged to turn; the first band and the second band being arranged to run part of the way in parallel such that they define between them a drying zone, the first band being heated and the second band being cooled, and the fiber web and at least two felts or wires, of which at least one is a coarse wire and at least one is a fine wire being arranged to run between the bands in such a manner that the fiber web is in contact with the first, heated band, the coarse wire is in contact with the second, cooled band, and the fine wire, correspondingly, is between the fiber web and the coarse wire.
The invention further relates to an apparatus for dying a fiber web, the apparatus comprising two endless bands that are impermeable to air, first turning rolls around which the first band is arranged to turn, and second turning rolls around which the second band is arranged to turn; the first band and the second band being arranged to run part of the way in parallel such that they define between them a drying zone, the first band being heated and the second band being cooled, and the fiber web and at least two felts or wires, of which at least one is a coarse wire and at least one is a fine wire being arranged to run between the bands in such a manner that the fiber web is in contact with the first, heated band, the coarse wire is in contact with the second, cooled band, and the fine wire, correspondingly, is between the fiber web and the coarse wire.
Many patent publications, such as WO 96/11300 and U.S. Pat. No. 4,461,095, teach the drying of a fiber web between two parallel metal bands moving in the same direction such that the fiber web is in contact with the heated metal band and there is a wire between the fiber web and the second, cooled metal band, whereby steam separated from the fiber web by heating condenses to the wire by the effect of the cold metal band. The basic idea is that two endless metal bands are arranged to turn around turning rolls and that against the inner surface of the loops formed by the bands are provided pressure chambers containing hot steam and water, respectively, such that the pressure produced presses the hot and cold bands, respectively, against the fiber web and the wire running between them. Together with seals, the bands located between the pressure chambers define one side of the pressure chambers, whereby the steam and the water can directly affect the bands. The operation of the apparatus is fully known per se and has been disclosed, for example, in the above patent publications, which are incorporated herein by reference.
The basic idea of the operation of the drying apparatus is that because of the heat of the heated band, the water in the web is vaporized and transferred through the wire or wires towards the cooled band. The steam produced on the surface of the cooled band condenses into water and is removed with the cooled band and the wire against it. U.S. Pat. No. 5,594,997 teaches the removal of water from the wire against a cooled band by means of blow and/or suction boxes. In the drying apparatuses described above, the fiber web is guided to and from the apparatus by means of a fine wire. The fine wire is placed between the coarse wire located against the cooled band and the fiber web, and the fine wire further aims to prevent the wiremark on the surface of the web caused by the coarse wire. The small apertures of the wire against the fiber web are susceptible to being clogged, which causes the wire to adhere to the surface of the fiber web. This, again, renders the surface of the web uneven. In addition, clogged apertures prevent the water from passing from the web to the coarse wire, resulting in a lowered drying efficiency of the web.
An object of the present invention is to provide a method and an apparatus which can avoid the above disadvantages.
The method of the invention is characterized by cleaning the fine wire.
Further, the apparatus of the invention is includes a cleaning device for cleaning the fine wire.
The essential idea of the invention is that a fine wire is arranged between the wire against the cooled band and the fiber web, and the fine wire is cleaned. In a preferred embodiment, the cleaning is performed by spraying water onto the wire by a water nozzle. In another preferred embodiment, after the water nozzles, warm or hot air is blown through the wire from the opposite side to the water nozzles. In a third preferred embodiment, after the water nozzle, warm or hot air is blown through the wire also from the same side of the water nozzle.
The advantage of the invention is that the small apertures of the wire can be kept unclogged and the wire does not adhere to the surface of the wire. Further, as the apertures are unclogged, water from the web passes easily through the wire; in all, the surface of the web can be made even. The water nozzles allow the cleaning to be performed extremely efficiently; the air blowing allows harmful moisture to be discharged from the wire. The moisture would also show in the moisture profile of the web and in the characteristics of finished paper and board. By blowing air also from the opposite side of the wire to the water nozzles allows moisture from the wire to be removed extremely efficiently.